Many wireless radio communication systems use one or more base stations and a plurality of transceivers that communicate with the base stations. The transmitting power of each transceiver must exceed a minimum level in order to achieve an acceptable signal-to-noise ratio at the respective receiving base station. The signal strength of a transceiver received at the receiving base station can be increased by increasing the transmitting power of each transceiver. However, when two or more transceivers are communicating with a base station in a shared radiation frequency bandwidth, the communication signals produced by the transceivers may interfere with one another and hence can degrade the signals received at the receiving base station. Therefore, there is an upper limit to the maximum transmitting power of each transceiver.
Other restrictions may also limit the upper limit of the transmitting power of a transceiver. For example, if the transmitting power of one transceiver is too high when communicating with one base station that is adjacent to another base station, an sufficient amount of the transmitting signal may be received by the adjacent base station to which the signal is not intended to send. This may adversely affect the operation of the adjacent base station. For another example, undesirable sidebands may be generated by modulation or nonlinear distortion due to operation at a high power. These sidebands may also adversely affect other base stations or even a different wireless radio communication system that operates at a different radio frequency.
A cellular phone system, including both analog and digital systems, is one example of the above wireless radio communication system. A digital cellular phone system such a code division multiple access ("CDMA") system, a time division multiple access ("TDMA") system, or a frequency division multiple access ("FDMA") system , for example, is often required to limit the transmitting power of each handset within a specified power range defined by predetermined maximum and minimum power levels. Such power range for handsets for a particular cellular system is specified by a respective cellular system standard. IS-95 and IS-54, for example, are the current US standards for CDMA and TDMA systems, respectively.
The transmitted power level of a handset can be preset at a desired level within a specified power range. However, the transmitted power is often subject to variation due to the temperature variation, frequency drift or other changing conditions. Such power variation may cause the transmitted power drift out of the specified range. Hence, it is desirable to implement a power control mechanism in a handset. For a given cellular system standard, different methods and circuits may be used to implement the specified power range.
U.S. Pat. No. 5,193,223 to Walczak and Cahill discloses one implementation of such power control for a TDMA cellular phone by repeatedly sampling and adjusting the transmitting power levels. For each sampled output power level, a desired power level and the actual transmitted power level is compared to determine the difference therebetween. The gain of a variable gain stage in the output signal path is then adjusted to reduce the difference.
One limitation of the above implementation is that a digital controller in the handset needs to respond to a sampled power level each and every time to produce a command that would change the gain of the variable gain stage in a desired manner. The digital controller is configured to do so even when the sampled transmitted power indicates that the transmitted power is at a desired value within the specified power range. This prevents the digital controller from performing other processing tasks or reduces the amount of processing power of the digital controller for processing other tasks during the period when the comparison of power levels is performed.